All free string theories are theories of BRST cohomology

The fully gauge-invariant theory of the free open (super) strings is generalized to treat closed (super) strings in the BRST formalism. It is shown that the actions for all theories can be written in the form (Φ,Q_BΦ) using the BRST charge Q_B and the string functional Φ, if the inner product is interpreted appropriately with necessary ghost insertions. Also is shown that consistent truncations of our actions recover all known covariant formulations of (super) strings.